Generally, the vehicle body frame of a track vehicle is manufactured by joining a roof structure, side structures that face each other, and a floor structure.
A method of manufacturing such a vehicle body frame is described in the following Patent Document 1, for example. In the technique described in this Patent Document 1, structures, such as side structures, are formed by a plurality of panel boards called double skin. The panel board has an inner face plate and an outer face plate that face each other, and a plurality of ribs that connect the inner face plate and the outer face plate. In the technique described in this Patent Document 1, a side structure is formed by joining outer face plates of a plurality of panel boards by friction-stir welding, and inner face plates of the plurality of panel boards by welding or the like. The side structures formed as described above, the roof structure, and the floor structure are joined to manufacture a vehicle body frame.